The substantial potential for high-power continuous wave and pulsed lasing in coherently coupled diode-laser arrays has led to considerable interest in their development. Attention has focussed on the problem of controlling the spatial emission characteristics of these devices with the aim of promoting single-lobed far-field emission patterns. The approaches most often used entail a `chriping` of the gain profile across the device by varying the stripe widths or the spacing between strips. Alternatively, such a spatially chirped gain has also been realized by fabrication of a diode-laser array structure in which the gain of each stripe could be independently controlled via separate current contacts. Although the above approaches have demonstrated some degree of success in producing single-lobed far-field patterns, they provide no improvement in the spactral characteristics of the devices, which in the case of gain-guided arrays, oscillate in several longitudinal modes.
Semiconductor diode-laser arrays provide an intense and efficient source of laser radiation. However, these devices have two inherent drawbacks which limit their usefulness. They generally emit their radiation in a two-lobed far-field pattern which makes beam focusing and/or propagation difficult, and the spectral distribution of their emission is spread over many angstroms. In addition, it is difficult to cause the output of the laser array to scan even a limited field.
Injection-locking is well known for controlling the spectral distribution of single-channel diode lasers. Furthermore, control of both the spectral and spatial distribution of diode-laser arrays has been demonstrated using a single-frequency diode laser as the master oscillator. In L. Goldberg, H. F. Taylor, J. F. Weller, and D. R. Scifres, "Injection-Locking of Coupled-Stripe Diode Laser Arrays", Applied Physics Letters, 46, 236 (1985), injection locking is applied to a diode laser array to control the spatial profile of light emitted by the array. Because all the elements of the array are injected uniformly, injection is applied to the array at an angle of 4 degrees off the array axis in order to produce a phase tilt across the array leading to emission in a single beam.
As the injection array frequency was varied, a frequency interval (locking bandwidth) was observed over which the single-lobed emission from the array could be maintained. When a single element of the array was injected (again at an angle of 4 degrees to the array axis), there was a much smaller locking range (less than 1 GHz). There was not, however, any reference to or realization of scanning of the emission beam from an injection-locked array by varying the injection frequency of the array.
Previously, it has been observed that by incorporating a semiconductor laser array into a cavity containing a grating as a tuning element, the emission angle of the two-lobed far-field beam changes as the grating is tuned. In this case the far-field beam consists of two-lobes of nearly equal intensity centered about the axis of the diode-array with the result that there was no net angular steering of the total beam. The prior art does not disclose scannable optical injection of radiation for steering the emission of a single-lobed far-field diode laser beam.
Various means of scanning laser beam output using the input frequency as a medium to bring about the scan are known. For example, in U.S. Pat. No. 3,541,471 to Kaminow et al, scanning is limited to a fixed frequency which requires the scanning frequency to be equal to the transverse mode separation frequency.
Generally in such systems, separate phase modulators and/or frequency shifters are required. These separate external or intracavity phase modulators and/or frequency shifters are undesirable since they introduce additional losses in the optical power available and require additional components and circuitry to monitor and control their performance.
In U.S. Pat. No. 3,691,483 to Klein, phase control is integrated into the semiconductor structure itself. However, the use of a separate electrical input signal to control the relative phase shift of each laser in the array and a computer to synchronize and control the overall phase tilt of the array are required.
In U.S. Pat. No. 3,626,321 to Smith and in U.S. Pat. No. 3,691,483 to Klein, generation of multiple input beams, together with associated mirrors or optical distributors as required to generate the multiple input beams, is disclosed.
In general, it is desirable to scale up the diode laser array to a very large number of emitting elements (in the form of a one- or two-dimensional array). This scaling is very difficult and costly, if not impossible, with the prior art. In U.S. Pat. No. 3,626,321 to Smith, for example, a plurality of laser beams are generated by means of interference effects, and the intensity of any one beam is therefore correspondingly reduced.
In U.S. Pat. No. 3,541,471 to Kaminow et al, the spatial intensity profile of the emitted beam varies with the scan angle since the selection of the transverse modes which are excited is angle depenent. Such dependence of spatial intensity of the emitted laser beam upon the scan angle is undesirable.